The major objective has been to characterize and evaluate the contribution of the high molecular weight hemoglobin, found in lysate obtained from methyl acetimidate treated sickle-cell erythrocytes, to the antisickling effect of methyl acetimidate. High molecular weight hemoglobin is extensively crosslinked and heterogeneous. Isolated high molecular weight hemoglobin has a high oxygen affinity, a low Hill constant, and does not gel when completely deoxygenated at a hemoglobin concentration of 37 g%. The antisickling effect of methyl acetimidate may be due both to an increase in oxygen affinity of hemoglobin in the treated cells and to a decrease in the intracellular concentration of hemoglobin S which is capable of participating in the formation of deoxyhemoglobin S gel fibers. However, since both the minimum gel concentration and the oxygen affinity of gel filtered lysate, containing no high molecular weight hemoglobin, are also increased (when compared to gel filtered lysate from control cells) the high molecular weight hemoglobin is not solely responsible for the antisickling effect observed in methyl acetimidate treated sickle-cell erythrocytes. The goals for the coming year are to continue to study the effects of the methyl acetimidate incubation on the physiological properties of the modified hemoglobin. Both the oxygen affinity of incubated cells and modified hemoglobin obtained from the modified erythrocytes will be determined as a function of the effect of 2,3-diphosphoglycerate and inositol hexaphosphate on the oxygen affinity and the reaction itself will also be studied.